Components are usually integrated into a system by securing the components to an assembly. There are many ways to secure components to an assembly. Often, components are clamped to a circuit board. As shown in FIG. 1, components 14, 18 are clamped to circuit board 20 to hold components 14, 18 in place when components 14, 18 are being soldered into place. Additionally, components are clamped to the circuit board rather than being soldered to the circuit board when it is difficult to provide a secure solder connection between the components and the circuit board or when it is useful to be able to rework the components. This is particularly advantageous for expensive components. For example, if a component is attached to a circuit board by solder, the entire circuit board has to be heated to melt the solder connection and remove the component. Clamping the component to the circuit board allows the component to be easily removed and replaced, without affecting the other components on the circuit board.
FIG. 2a shows assembly 50 having component 52 clamped to circuit board 54 with clamp 56. Typically, circuit board 54 is connected to back plate 58. Clamp 56 has rigid metal bar 72, and more compliant bar 74, for example fabricated from FR4. Bars 72, 74 are fastened to circuit board 54 and back plate 58 with screws 76, 78. Circuit board 54 has an opening through which the bottom of component 52 contacts back plate 58. Referring to FIGS. 2a and 2b, component 52 is typically, one, or a plurality of semiconductor devices or integrated circuits (ICs) 60 housed in plastic or ceramic package 62. Package 62 has walls 77, 79 and lid 80. There is a thin conductive layer on the bottom of package 62 referred to as a flange. Flange 64 is electrically coupled to the bottom of device or IC 60. Leads 68, 70 of component 52 contact circuit board 54. Clamp 56 has to apply enough force to component 52 to press component 52 tightly enough to the rest of assembly 50 to ensure that there is a sufficient electrical connection between flange 64 and back plate 58 and between leads 68, 70 and circuit board 54. Screws 76, 78 hold bars 72, 74 tightly to component 52 so that bars 72, 74 apply force to component 52.
A problem with using clamp 56 to attach component 52 to circuit board 54 and back plate 58 is that when clamp 56 applies enough force to component 52 to ensure sufficient electrical connection a significant amount of force is asserted against lid 80. For example, in an RF transistor 250 to 300 lbs./sq.in. of force may be needed to ensure sufficient electrical connection. The thin lid of the package of many components is too thin to withstand this much force, causing the package to crack.
Furthermore, the force exerted by clamp 56 varies based on the tolerances of screws 76, 78 and the coefficient of friction between screws 76, 78 and back plate 58. This force will vary from clamp to clamp and screw to screw. Therefore, to ensure a sufficient electrical connection in the worst case, more force than the 250 to 300 lbs./sq.in. has to be applied. This force is often large enough to crack lid 80 of many components.
Another problem with using clamp 56 to attach component 52 to circuit board 54 and back plate 58 is that the tolerances of package 62 do not ensure that clamp 56 will evenly press against component 52 to ensure a sufficient electrical connection. The tolerances in manufacturing package 62 allow for a package that is not completely flat. When clamp 56 presses against component 52, because metal bar 72 is rigid, gap 81 forms between bar 74 and component 52, even though bar 74 is somewhat compliant. Clamp 56 does not apply enough force to the portion of component 52 under gap 81. This may cause an insufficient electrical connection between the portion of flange 64 under gap 81 and back plate 58, so component 52 does not function acceptably.
To get a better electrical connection, more force can be applied on component 52 by clamp 56. However, now the force on taller side 82 of the package is even greater. Often this force is too great for lid 80 of many components to withstand, causing the package to crack and rendering component 52 useless.
Yet another problem with fastening component 52 to assembly 50 is that when clamp 56 is being fastened it can easily crack component 52 as shown in FIG. 3. Clamp 56 can be fastened to assembly 50 by either first tightening screw 76 fully into place and then tightening the other screw, or by gradually tightening each screw while constantly alternating between the two screws. Tightening screw 76 fully places all of the force of clamp 56 on corner 82 of component 52, crushing corner 82. This can cause a crack to start in corner 82, which will then spread to the rest of the top of the package of component 52, rendering component 52 useless. Gradually tightening each screw while constantly alternating between the two screws reduces this problem, however, it is much more time and labor intensive, which greatly increases the time and cost of the assembly.